The present invention relates to a pneumatic tire having an asymmetric tread pattern wherein a right and a left half of a tread portion on both sides of the tire equator have different patterns, and more particularly to a pneumatic tire that a difference in performances depending on the rotation direction of the tire is reduced.
Various tread patterns formed by tread grooves, including a circumferential groove extending in the circumferential direction of the tire and a lateral groove extending in a direction intersecting with the circumferential groove, have been proposed in order to enhance a wet performance of tires. As one type of the tread patterns is known, for example, an asymmetric pattern such that patterns on the both sides of the tire equator are different, as disclosed in JP-A-2004-155416.
A tire with an asymmetric pattern has the advantage that both dry and wet performances can be achieved at high levels, since a land proportion for an outer area of the tread surface which lies on a vehicle outer side with respect to the tire equator when the tire is fitted to a vehicle can be made larger than a land proportion for an inner area of the tread surface which lies on a vehicle inner side with respect to the tire equator. The reason is that the vehicle outer side area has a large influence on the steering stability since a load applied to the vehicle outer side area becomes relatively large as compared with the vehicle inner side area at the time of cornering or lane change and, therefore, the dry performances can be improved by increasing the land proportion of the vehicle outer side area to thereby enhance the pattern rigidity and to increase the ground contact area, while maintaining the wet performances as a whole tire, particularly drainage performance, at a high level by the vehicle inner side area which has a smaller land proportion.
However, as shown exaggeratingly in FIG. 6, in the case that tires having such an asymmetric pattern are fitted to a vehicle, the tread pattern is directed in opposite directions with respect to the rotation direction of the tires between a tire T1 on a right wheel and a tire T2 on a left wheel. That is, explaining with reference to FIG. 6, lateral grooves a1 in the outer area of the right wheel tire T1 extend from a tire equator C side toward a tread edge Te side and are inclined in a backward direction with respect to the rolling direction of the tire, whereas lateral grooves a1 in the outer area of the left wheel tire T2 extend from the tire equator C side toward the tread edge Te side and are inclined in a forward direction with respect to the tire rolling direction.
In this case, the right wheel tire T1 can exhibit a high drainage performance, since water on a road flows in a direction of discharge from the tread edge Te. In contrast, the left wheel tire T2 is inferior in the drainage performance, since water on a road flows toward the tire equator C. Like this, tires with an asymmetric pattern have a problem that as to the drainage performance, a difference may occur due to inclination of lateral grooves between the tire mounted on the right wheel and the tire mounted on the left wheel.
In particular, since the vehicle inner side area has generally a small land proportion, in other words, since the vehicle inner side area has a large groove area and accordingly exhibits a high drainage performance, an influence of the vehicle inner side area onto a difference in the drainage performance between the right and left wheel tires is small, but the vehicle outer side area has generally a large land proportion and accordingly has a large influence on the difference in the drainage performance between the right and left wheel tires.
Accordingly, it is an object of the present invention to provide a pneumatic tire which has an asymmetric tread pattern such that the land proportion of the vehicle outer side area is larger than the land proportion of the vehicle inner side area, but which is capable of suppressing occurrence of a difference in drainage performance between the tire fitted to a right wheel and the tire fitted to a left wheel of a vehicle to improve the steering stability of the vehicle.
This and other objects of the present invention will become apparent from the description hereinafter.